updated ApiClient of python sdk.

When create api instance, if we don't pass the `api_client` parameter,
then use the default api_client in `config` module.

samples/client/petstore/python/SwaggerPetstore-python/tests/test_pet_api.py

@@ -13,6 +13,7 @@ import unittest
 
 import SwaggerPetstore
 from SwaggerPetstore.rest import ErrorResponse
+from SwaggerPetstore import config
 
 HOST = 'http://petstore.swagger.io/v2'
 
@@ -49,6 +50,26 @@ class PetApiTests(unittest.TestCase):
         self.test_file_dir = os.path.realpath(self.test_file_dir)
         self.foo = os.path.join(self.test_file_dir, "foo.png")
 
+    def test_create_api_instance(self):
+        pet_api = SwaggerPetstore.PetApi()
+        pet_api2 = SwaggerPetstore.PetApi()
+        api_client3 = SwaggerPetstore.ApiClient()
+        api_client3.user_agent = 'api client 3'
+        api_client4 = SwaggerPetstore.ApiClient()
+        api_client4.user_agent = 'api client 4'
+        pet_api3 = SwaggerPetstore.PetApi(api_client3)
+
+        # same default api client
+        self.assertEqual(pet_api.api_client, pet_api2.api_client)
+        # confirm using the default api client in the config module
+        self.assertEqual(pet_api.api_client, config.api_client)
+        # 2 different api clients are not the same
+        self.assertNotEqual(api_client3, api_client4)
+        # customized pet api not using the default api client
+        self.assertNotEqual(pet_api3.api_client, config.api_client)
+        # customized pet api not using the old pet api's api client
+        self.assertNotEqual(pet_api3.api_client, pet_api2.api_client)
+
     def test_add_pet_and_get_pet_by_id(self):
         self.pet_api.add_pet(body=self.pet)